I. Field
The following description relates generally to wireless communications, and more particularly to methods and apparatuses that facilitate estimating Doppler spread.
II. Background
Wireless communication systems are widely deployed to provide various types of communication content such as voice, data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users by sharing the available system resources (e.g., bandwidth and transmit power). Examples of such multiple-access systems include code division multiple access (CDMA) systems, time division multiple access (TDMA) systems, frequency division multiple access (FDMA) systems, 3GPP Long Term Evolution (LTE) systems, and orthogonal frequency division multiple access (OFDMA) systems.
Generally, a wireless multiple-access communication system can simultaneously support communication for multiple wireless terminals. Each terminal communicates with one or more base stations via transmissions on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. This communication link may be established via a single-in-single-out, multiple-in-signal-out or a multiple-in-multiple-out (MIMO) system.
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit and NR receive antennas may be decomposed into NS independent channels, which are also referred to as spatial channels, where NS≦min {NT, NR}. Each of the NS independent channels corresponds to a dimension. The MIMO system can provide improved performance (e.g., higher throughput and/or greater reliability) if the additional dimensionalities created by the multiple transmit and receive antennas are utilized.
A MIMO system supports a time division duplex (TDD) and frequency division duplex (FDD) systems. In a TDD system, the forward and reverse link transmissions are on the same frequency region so that the reciprocity principle allows the estimation of the forward link channel from the reverse link channel. This enables the access point to extract transmit beamforming gain on the forward link when multiple antennas are available at the access point.
With respect to mobility of a mobile device, it is noted that such mobility typically introduces Doppler shift and Doppler spread to a received signal. For LTE systems, for example, the mobile speed can be supported for up to 350 kilometers/hour, which introduces relatively high Doppler shift/spread as well as temporal variation of the received signal. At the receiver, although conventional systems can partially compensate for Doppler shift by implementing a frequency tracking loop, Doppler spread is typically more difficult to compensate. In addition, fast channel variation has to be taken into account for both scheduler as well as receiver algorithms. For example, in a channel estimation block, one can use the mobile speed information to determine whether channel interpolation or pilot averaging should be used.
In general, it is noted that conventional Doppler estimation is usually based on correlation of pilot symbols, which generally requires more than one pilot symbol per estimate. However, such a requirement is difficult to meet in current the uplink frame structure of LTE systems. For Physical Uplink Shared Channel (PUSCH) with intra-Transmission Time Interval (TTI) frequency hopping, for example, there is only one pilot per slot. Also, for Physical Uplink Control Channel (PUCCH)-Channel Quality Indication (CQI) channel, although two pilot symbols exist for normal cyclic prefix (CP), there is only one pilot symbol in extended CP. Likewise, for PUCCH-Acknowledgment channel, due to time-domain spreading, there is also only one pilot/channel estimate after de-spreading. Accordingly, the current standard does not allow for such a conventional solution.
The above-described deficiencies of current wireless communication systems are merely intended to provide an overview of some of the problems of conventional systems, and are not intended to be exhaustive. Other problems with conventional systems and corresponding benefits of the various non-limiting embodiments described herein may become further apparent upon review of the following description.